Counting cells happens by forcing them to pass through a small tunnel, causing disruptions in a laser light beam or electricity flow. These disruptions are being analyzed by the instruments. It is hard to create these small tunnels for these cells using ordinary manufacturing processes, as the diameter should be in the magnitude of micrometers, and the length of the tunnel should exceed several millimeters.
Focusing with a fluid
Hydrodynamic focusing solves this by building up the walls of the tunnel from fluid, using the effects of fluid dynamics. There is a wide (hundreds of micrometers in diameter) tube created of glass or plastic, and a "wall" fluid called the sheath fluid is being pumped through. The sample is injected into the middle of the sheath flow. If the two fluids differ enough in their velocity or density, they do not mix: they form a two-layer stable flow, with the sheath enveloping the sample in a linear, steady flow. In effect, the sample flow cross-section can be much smaller than the solid tunner around it, even down to the micrometer magnitude.